Use of final Keyword in C++

final keyword specifies that a virtual function cannot be overridden in a derived class. It also specifies that a class cannot be inherited from. It ensures that the function is virtual, otherwise a compile-time error is generated. Similarly when used in a class definition, it ensures that class cannot be used as base class by any other class.

Use of final in Class

In the below example Base is a virtual class. Structure B is declared as final, so inheriting of this structure by structure C will give compilation error.

struct Base
{
    virtual void foo();
};
 
struct B final : Base // struct B is final
{
    void foo() final;
};
 
struct C : B // Error: B is final
{

};

To resolve the error, either B should be declared as non final or C should not inherit structure B.

Use of final in Function

In the below example, structure A is overriding the function foo using final keyword. So if structure B tries to override the function, compiler will give error.

struct Base {
    virtual void foo();
};
 
struct A : Base {
    void foo() final; // Base::foo is overridden and A::foo is the final override
};
 
// struct B is final
struct B final : A {
    void foo() override; // Error: foo cannot be overridden as it is final in A
};

So final is also used to mark a virtual function so as to prevent it from being overridden in the derived classes.